1. Field of the Invention
The present invention is directed to a detector for an X-ray computed tomography apparatus of the type having a number of detector elements of different widths separated from one another by septa, forming a detector line arranged in a direction of a rotational axis of the tomography apparatus, with a number of such detector lines being arranged next to each other, and wherein a predetermined number of channels are provided for acquiring signals generated by the detector elements, with one or more of the detector elements being selectively connectable to respective ones of the channels for acquiring the signals.
2. Description of the Prior Art
Detectors of the above general type are disclosed in German OS 195 02 574 and PCT Application WO 98/05980. They have a number of parallel detector lines that proceed in the direction of the axis of a subject to be transirradiated, for example a patient. Each detector line is composed of a number of detector elements that, for example, are manufactured of a scintillator ceramic.
For readout of the signals generated by the detector elements, usually four, currently at most eight, channels are available dependent on the computing power of the computer. Dependent on the demands regarding the desired image information, a number of detector elements lying next to one another in a detector line can be respectively connected to one channel. When, for example, two or more detector elements of a detector line are connected to one channel, information about a relatively large volume excerpt of the transirradiated subject is obtained. Such an information is especially well-suited for producing high-contrast images with which, for example, it is possible to differentiate soft parts in the brain.
When, in contrast, only one detector element from each line is respectively connected to each of the channels, the transirradiated volume is small. The information thus obtained is especially well-suited for the resolution of fine structures, for example in the inner ear region.
For gating the required, fan-shaped X-ray beam, a diaphragm precedes the known detector. The provision of such a diaphragm causes increased manufacturing outlay. Moreover, the detector has a number of detector elements with a number of septa separating them. The efficiency of such a detector is not especially high. The multitude of provided detector elements further increases the manufacturing outlay of the detector.
An object of the invention is to provide a simply constructed, universal detector with enhanced efficiency for an X-ray computed tomography apparatus.
This object is achieved in a detector of the type initially described wherein the septa are arranged at both sides of a symmetry plane according to the relationship:             {              s        k            }        =                  ⋃                  m          =          1                j            ⁢              {                              ⋃                          p              =                                                -                  z                                /                2                                                    z              /              2                                ⁢                      {                          p              ·                              n                m                            ·                              D                1                                      }                          }              ,
wherein {Sk} is the total number of septum locations,
z xcex5 N is the number of channels,
D1 is the smallest width of a detector element,
Di=ni D1 is the width of the ith subdivision, whereby i greater than 1 and ni greater than nixe2x88x921apply, and
j xcex5 N is the number of different widths Di.
The term xe2x80x9csmallest width D1xe2x80x9d mean the width of the detector element in the direction of the axis of the subject to be transirradiated, this axis generally proceeding parallel to the rotational axis of the detector. A channel is a slice of detector elements that is simultaneously processed by the measuring electronics.
The inventive relationship supplies a set of septum positions proceeding from the symmetry plane. This set corresponds to the union of sets of the septum locations for a predetermined number of widths Di the predetermined widths Di of the detector elements as well as the predetermined number z of channels.
The solution of the equation produces a minimal number of septum locations. A detector manufactured according to the inventive relationship is thus distinguished by a number of septa that is minimized for the respective application. The efficiency of such a detector is enhanced. A diaphragm at the detector can be foregone.
It is advantageous for the smallest width D1 to exhibit one of the following values: g 0.5 mm, g 0.625 mm, g 1.0 mm or g 1.25 mm, whereby g is a geometry factor to which the following applies:   g  =            Spacing      ⁢              xe2x80x83            ⁢              (                              tube            ⁢                          xe2x80x83                        ⁢            focus                    -                      detector            ⁢                          xe2x80x83                        ⁢            surface                          )                    Spacing      ⁢              xe2x80x83            ⁢              (                              tube            ⁢                          xe2x80x83                        ⁢            focus                    -                      rotational            ⁢                          xe2x80x83                        ⁢            center                          )            
The term xe2x80x9cspacing (tube focusxe2x88x92detector surface)xe2x80x9d means the distance from the focus of the X-ray tube up to the surface of the detector. The term xe2x80x9cspacing (tube focusxe2x88x92rotational center)xe2x80x99 means the distance from the focus of the X-ray tube to the rotational axis of the detector.
In another embodiment that a detector line has detector elements with at least three different widths.